Boosted braking device with emergency valve

ABSTRACT

Braking device, for a motor vehicle, comprising: a master cylinder ( 2 ), a primary piston ( 3 ) mounted to slide in the master cylinder, a manual-control member ( 4 ), a booster coupled to the manual-control member, an emergency assist valve (VA) comprising a reaction piston ( 17   a ) and a rapid piston ( 17   b ), and a ratio control (T) actuated by a plunger distributor ( 10 ) driven by the manual-control member ( 4 ). The reaction piston ( 17   a ) and the rapid piston ( 17   b ) form one and the same stepped piston ( 17 ) having a part ( 17   a ) of large cross section and a part ( 17   b ) of small cross section, the large-section part ( 17   a ) determining, with a bore ( 18 ) of the primary piston ( 3 ), a variable-volume annular chamber ( 27 ). Separation/communication means ( 28, 29, 30 ), controlled by the displacement of the stepped piston ( 17 ), are designed so that the pressure of the liquid is exerted effectively on the large cross section ( 17   a ) of the stepped piston when the latter occupies its position of rest or is to the rear of this position, and on only the small cross section ( 17   b ) when the stepped piston is displaced forward relative to the main piston under emergency braking.

[0001] The invention relates to a boosted braking device, for a motorvehicle, of the kind which comprise a master cylinder controlling thepressure in at least one brake circuit; a primary piston mounted toslide in the master cylinder to create therein a variation in pressure,this primary piston being subjected to an actuating force made up of aninput force exerted by a manual-control member and of a boost forceexerted by a booster which is coupled to the manual-control member.

[0002] The booster may be pneumatic and comprise a rigid casing dividedinto two pneumatic chambers by a moving partition which may be subjectedto a difference in pressure between the chambers under the action of avalve actuated by the manual-control member.

[0003] A braking device of this type is known, for example, from EP-B-0662 894.

[0004] The conditions under which braking is exerted may differ. A firstinstance corresponds to ordinary conditions when an obstacle is seenfrom a distance and braking is performed relatively gently; this type ofbraking is known as “normal braking” or “slow braking”. Another instanceis that of abrupt braking or “emergency braking”, for example when anobstacle appears suddenly in front of the driver who has to bring hisvehicle to rest as quickly as possible.

[0005] With a view to satisfying these various braking conditions, aboosted braking device, of the type defined previously, has beensupplemented by an emergency assist valve capable of initiating theintervention of at least two boost ratios corresponding respectively toslow braking and to emergency braking; the boost ratio for slow brakingis lower and the hydraulic reaction opposing the advancement of themanual-control member is greater. In the case of emergency braking, theboost ratio is stronger and the hydraulic reaction against themanual-control member is weaker, which means that the driver can brakefor longer and more heavily.

[0006] By convention, the term “front” will be used in the remainder ofthe text to mean a direction directed from the control member toward themaster cylinder, and the term “rear” or “back” will be used to denotethe opposite direction.

[0007] The emergency assist valve comprises a reaction piston whichslides in a sealed manner in a bore of the primary piston, the frontpart of this bore communicating with the interior volume of the mastercylinder, a rapid piston of cross section smaller than that of thereaction piston sliding in a sealed manner in a bore of correspondingdiameter of the primary piston, and a ratio control actuated by aplunger distributor itself driven by the manual-control member, theassembly being arranged in such a way that under emergency braking, thehydraulic reaction is exerted only on the small cross section of thisrapid piston. The slow boost ratio involves the larger-section reactionpiston.

[0008] A braking device such as this with an emergency assist valve isentirely satisfactory from the operational and braking force point ofview. However, embodiments hitherto proposed for the emergency assistvalve are relatively bulky with a relatively high number of parts givingrise to a not insignificant cost of manufacture.

[0009] It is an object of the invention, above all, to provide a brakingdevice with a more compact emergency assist valve and which has a lowercost of manufacture.

[0010] According to the invention, a boosted braking device, for a motorvehicle, of the type defined previously, comprising an emergency assistvalve, is characterized in that the reaction piston and the rapid pistonform one and the same stepped piston having a part (reaction piston) oflarge cross section and a part (rapid piston) of small cross section,the large-section part determining, with the corresponding bore of theprimary piston, an annular chamber, the volume of which varies accordingto the displacement of the stepped piston relative to the primarypiston, and that separation/communication means, controlled by thedisplacement of the stepped piston are designed so that the pressure ofthe liquid is exerted effectively on the large cross section of thestepped piston when the latter occupies its position of rest or is tothe rear of this position, and on only the small cross section when thestepped piston is displaced forward relative to the primary piston underemergency braking.

[0011] The large-section part of the stepped piston may lie toward thefront and the small-section part toward the rear. The small-section partmay have a shoulder against which there bears axially a washer acting asa thrust washer for a compression spring, the other end of which bearsagainst a stop piece anchored in a housing of the primary piston.

[0012] The means of separation between the large and small cross sectionof the stepped piston may be connected to the primary piston. A blindbore is advantageously provided in the stepped piston and open forward,this blind bore communicating, toward its interior end, via at least onehole, with the periphery of the small-section piston, while a sealingmeans, connected to the primary piston, is provided in the annularchamber, around the small-section piston, to collaborate with thehole(s) in the small-section piston. When the hole(s) is (are) to therear of the sealing means, the front part of the annular chamber isisolated from the hydraulic pressure of the master cylinder, which meansthat this pressure acts effectively on the entire area of the largecross section, whereas when the hole(s) is (are) in front of the sealingmeans, the hydraulic pressure is exerted in the front part of theannular chamber in such a way that this pressure is effective on onlythe small cross section. The sealing means advantageously consists of alip seal.

[0013] As a preference, the small-section piston is extended toward theplunger distributor by a rod of smaller diameter forming the ratiocontrol. There may be a gap at rest between the rear end of the rod andthe plunger distributor.

[0014] Apart from the provisions set out hereinabove, the inventionconsists in a certain number of other provisions which will be dealtwith more explicitly hereinafter with regard to an exemplary embodimentdescribed with reference to the appended drawings but which is not inany way limiting. In these drawings:

[0015]FIG. 1 is a part view in section with partial cutaway of a brakingdevice according to the invention;

[0016]FIG. 2 is a part view in section, on a larger scale, of elementsof FIG. 1 and of the emergency assist valve, the braking device being inthe position of rest;

[0017]FIG. 3, finally, shows, in a similar way to FIG. 2, the elementsat the onset of rapid braking.

[0018] The overall structure and general operation of a boosted brakingdevice of the type of the invention are known, particularly from patentsEP-B-0 662 894 or FR-B-2 658 466 and only a brief reminder thereof willbe given. For further details, reference may be made to the twoaforementioned patents which are incorporated into the description byreference.

[0019]FIG. 1 shows a braking device 1 for a motor vehicle, whichcomprises a master cylinder 2, partially depicted, and a primary piston3 mounted to slide in the master cylinder 2. A manual-control member 4comprising a linkage coaxial with the primary piston 3 is designed toexert on this piston an input force from back to front, that is to sayfrom right to left according to the depiction of FIG. 1. The forwarddisplacement of the primary piston 3 creates an increase in pressure ofthe liquid in the interior volume 5 of the master cylinder, connected toat least one hydraulic brake circuit. The control member 4 is generallyactuated by a brake pedal 4 a, depicted schematically.

[0020] A pneumatic booster 6 is coupled to the control member 4. Thebooster 6 comprises a rigid casing 7 divided internally in a sealedmanner into two pneumatic chambers 7 a, 7 b by a moving partition 8comprising an elastomer diaphragm 8 a and a rigid skirt 8 b. The chamber7 a is permanently connected to a source of vacuum (not depicted) by anozzle A. A pneumatic piston 9 in the form of a sleeve coaxial with theprimary piston 3 is fixed to the rigid skirt 8 b. The pneumatic piston 9is mounted to slide, in a sealed manner, on a plunger distributor 10, ofcylindrical overall shape. Sealing is provided by an O-ring 11.

[0021] The plunger distributor 10 comprises, toward the front, a head 10a and, toward the rear, a blind axial housing 10 b, open toward therear, which receives a bore 4 b provided at the end of the linkage 4.The plunger distributor 10 comprises, on the opposite side to the piston3, a frustoconical widening 12 which bears in a sealed manner against anelastomer ring 13 connected in terms of axial translation to thepneumatic piston 9. The assembly 12, 13 constitutes part of a three-wayvalve B (depicted partially) which either allows the chamber 7 b to beisolated from the atmosphere and the chambers 7 a, 7 b to be placed incommunication or allows the chambers 7 a, 7 b to be isolated from oneanother and air at atmospheric pressure to be let into the chamber 7 bwhen the widening 12 moves axially away from the ring 13.

[0022] The pneumatic piston 9 is returned toward its position of rest,depicted in FIG. 1, by a compression spring 14 arranged between thepiston 9 and the wall opposite belonging to the casing 7 to which themaster cylinder 2 is connected. A cup 15 with a central opening 16 bearsaxially via its exterior peripheral edge against a shoulder of thepneumatic piston 9. The spring 14 presses the edge of this cup againstthe piston 9. The cup 15 bears axially, via the interior edgesurrounding its opening 16, against the rear end of the primary piston3. The outside diameter of the head 10 a is smaller than the diameter ofthe opening 16.

[0023] An emergency assist valve VA is designed to initiate theintervention of at least two boost ratios corresponding respectively tonormal (slow) braking and to emergency braking.

[0024] The valve VA comprises a single stepped piston 17 having a part17 a of large cross section situated toward the front and constituting areaction piston, and a part 17 b of small cross section, situated at therear, constituting a rapid piston.

[0025] The part 17 a is mounted to slide in a sealed manner by virtue ofan O-ring, in a bore 18 of the primary piston 3. The front part 18 a ofthis bore communicates with the interior volume 5 of the mastercylinder. A helical compression spring 19 bears against the front faceof the part 17 a and against a split ring 20 anchored in a groove of thebore 18, in front of the part 17 a. The spring 19 pushes the steppedpiston 17 backward. The rear part 17 b of the stepped piston slides in abore 18 b of corresponding diameter, in a sealed manner by virtue of anO-ring. The bore 18 b is coaxial with the main bore 18 which it extendsbackward. The entry to the bore 18 b has a frustoconical chamfer 18 c.The part 17 b is extended, backward, by a coaxial rod T of smallerdiameter, forming the ratio control. A shoulder 21 marks the transitionbetween the part 17 b and the rod T. The rod T passes through a bore 22,of diameter greater than the bore 18, which follows on from the bore 18b backward and opens toward the head 10 a against which the rod T canbear. At rest, there may be an axial clearance of the order of onemillimeter or several millimeters between the rear end of the rod T andthe head 10 a. The transition between the bore 18 b and the bore 22 ismarked by a radial shoulder 23 forming the end of the bore 22. A washer24 is engaged around the rod T. This washer 24 has a central hole, thediameter of which is equal, give or take the sliding clearance, to thatof the rod T. The edge of the interior hole of the washer 24 may bestopped by the shoulder 21. The outside diameter of the washer 24 issmaller than the diameter of the bore 22.

[0026] A helical compression spring 25 is arranged between the washer 24and a rear stop piece formed by a clip-in ring 26, for example made ofplastic, anchored in the interior wall of the bore 22.

[0027] The large-section part 17 a of the stepped piston 17 determines,with the end 18 d of the large-section part of the bore 18, an annularchamber 27, the volume of which varies according to the displacement ofthe stepped piston 17 relative to the primary piston 3.

[0028] A blind bore 28 is pierced axially in the stepped piston 17 andopens on the same side as the master cylinder 2, following an entry 28 aof larger diameter.

[0029] The blind bore 28 is extended into the small-section part 17 b ofthe piston 17. Near the end of the bore 28, radially-directed holes 29place the blind bore 28 in communication with the periphery of the part17 b. The length of the bore 28 and the axial position of the holes 29is such that when the piston 17 occupies the position of rest depictedin FIG. 1 and FIG. 2, the holes 29 are just in front of the end 18 d ofthe large-section part of the bore 18.

[0030] A lip seal 30 with sufficient elasticity and sufficient rigidity,for example made of elastomer, is housed in an annular groove 31provided in the bore 18 of the primary piston 3. The seal 30, connectedto the piston 3, bears axially against the end 18 d. The lip 32 of theseal 30 determines a frustoconical surface constituting a kind of funnelshape, the small base of which faces toward the end 18 d. The lip 32bears in a sealed manner against the exterior surface of thesmall-section part 17 b of the stepped piston. The rear end of the lip32 is in front of the holes 29 when the stepped piston 17 is in theposition of rest of FIGS. 1 and 2.

[0031] The lip seal 30 is designed to withstand the hydraulic pressurethat may be exerted behind it and to prevent this pressure from beingtransmitted to that part of the chamber 27 which lies in front of theseal 30. By contrast, if the pressure in the front part of the chamber27 increases, the lip 32 of the seal 30 can deform and move radiallyaway from the part 17 b to allow liquid to pass and allow the pressureto drop.

[0032] A forward displacement of the stepped piston 17 relative to theprimary piston 3 causes the holes 29 to move from behind to in front ofthe zone of contact of the lip 32 with the exterior surface of the part17 b.

[0033] The assembly made up of the lip seal 30 and the holes 29constitutes a kind of non-return valve, allowing liquid to flow in onedirection and impeding this flow in the other direction. The seal 30 andthe holes 29 are one non-limiting example of means ofseparation/communication between the large-section part 17 a and thesmall-section part 17 b of the piston 17 with feed or drainage of theannular chamber 27 controlled by the displacement of the stepped piston17. Any device equivalent to the combination of the seal 30 and theholes 29 may be used.

[0034] That being the case, the way in which the braking device worksremains conventional and only a brief reminder thereof will be given.

[0035] In the case of normal braking, the manual-control member 4 isdisplaced, relatively slowly, to the left. The valve B is actuated andfirst of all cuts off communication between the chambers 7 a and 7 b,then the frustoconical widening 12 moves away from the elastic ring 13,which allows air at atmospheric pressure to be let into the chamber 7 bof the pneumatic booster 6. The moving partition 8 transmits the boostforce to the pneumatic piston 9, which moves forward against the actionof the spring 14. The piston 9 drives the cup 15, which pushes theprimary piston 3 forward. The pressure of the liquid increases in thevolume 5 and in the bore 18 and in the volume lying behind the seal 30,this volume communicating via the holes 29 with the blind bore 28 andthe bore 18. The seal 30 prevents pressure from being transmitted to thefront part of the chamber 27.

[0036] Liquid pressure on the large-section part 17 a of the piston 17generates a backward force. When this force reaches and exceeds thepreload of the spring 25, the stepped piston 17 retreats, pushing thewasher 24 against the action of the spring 25 until such time as the rodT comes into abutment against the head 10 a.

[0037] This phase corresponds to a jump phase, during which no reactionopposes the advancement of the member 4, whereas a reduction in thevolume of the chamber 27 occurs; any possible increase in the pressurein this chamber 27 is limited because the lip 32 allows backward flow.

[0038] Under the action of the input force on the member 4, the plungerdistributor 10 continues its forward displacement; the pneumatic piston9 follows the plunger distributor 10 and pushes the primary piston 3with the boost force.

[0039] When saturation (maximum boost force) is reached, the manualinput force exerted on the member 4 is transmitted mechanically to theprimary piston 3 and allows the braking pressure to increase.

[0040] Under normal braking, the holes 29 remain behind the lip 32. Thepressure of the liquid in the bore 18 is exerted over the entire crosssection of the part 17 a because the seal 30 prevents a back-pressurefrom arising in the rear annular zone of the head 17 a around the part17 b. The hydraulic reaction opposing the advancement of the plungerdistributor 10 and of the member 4 is relatively strong.

[0041] Under rapid braking (FIG. 3), the plunger distributor 10 isinitially displaced more rapidly than the pneumatic piston 9 and theprimary piston 3. The head 10 a comes into contact with the rod T andpushes on it. The stepped piston 17 is displaced forward with respect tothe primary piston 3, compressing the spring 19.

[0042] The holes 29 will move from behind to in front of the lip 32which means that the pressure of the liquid in the bore 18 istransmitted, via the holes 29, to the part of the chamber 27 lying infront of the seal 30.

[0043] In this way, the hydraulic pressure will be effective, that is tosay will exert backward thrust, only on the small cross section 17 b.The reaction on the stepped piston 17 is thus lessened. This reactionwhich is weaker than it was under normal braking, makes it possible toobtain a high braking pressure in a minimum length of time.

[0044] The emergency assist valve VA according to the invention iscompact and contains a low number of parts, which makes it possible forits cost of manufacture to be reduced.

1. Boosted braking device, for a motor vehicle, comprising: a mastercylinder (2) controlling the pressure in at least one brake circuit; aprimary piston (3) mounted to slide in the master cylinder to createtherein a variation in pressure, this primary piston being subjected toan actuating force made up of an input force exerted by a manual-controlmember (4) and of a boost force exerted by a booster (6) which iscoupled to the manual-control member; an emergency assist valve (VA)comprising a reaction piston (17 a) which slides in a sealed manner in abore (18) of the primary piston, the front part (18 a) of this borecommunicating with the interior volume (5) of the master cylinder, arapid piston (17 b) of cross section smaller than that of the reactionpiston sliding in a sealed manner in a bore (18 b) of correspondingdiameter of the primary piston, and a ratio control (T) actuated by aplunger distributor (10) itself driven by the manual-control member (4),the assembly being arranged in such a way that under emergency braking,the hydraulic reaction is exerted only on the small cross section of therapid piston, characterized in that the reaction piston (17 a) and therapid piston (17 b) form one and the same stepped piston (17) having apart (17 a) of large cross section and a part (17 b) of small crosssection, the large-section part (17 a) determining, with thecorresponding bore (18) of the primary piston, an annular chamber (27),the volume of which varies according to the displacement of the steppedpiston (17) relative to the primary piston (3), and thatseparation/communication means (28, 29, 30), controlled by thedisplacement of the stepped piston (17) are designed so that thepressure of the liquid is exerted on the large cross section (17 a) ofthe stepped piston when the latter occupies its position of rest or isto the rear of this position, and on only the small cross section (17 b)when the stepped piston is displaced forward relative to the primarypiston (3) under emergency braking.
 2. Braking device according to claim1, characterized in that the separation/communication means comprise ameans of separation (30) between large and small cross section,connected to the primary piston (3).
 3. Braking device according toclaim 1, characterized in that the separation/communication meanscomprise a blind bore (28) provided in the stepped piston and openforward, this blind bore communicating, toward its interior end, via atleast one hole (29), with the periphery of the small-section piston (17b), while a sealing means (30), connected to the primary piston (3), isprovided in the annular chamber (27), around the small-section piston(17 b), to collaborate with the hole(s) (29) in the small-sectionpiston.
 4. Braking device according to claim 2, characterized in thatthe separation/communication means comprise a blind bore (28) providedin the stepped piston and open forward, this blind bore communicating,toward its interior end, via at least one hole (29), with the peripheryof the small-section piston (17 b), while a sealing means (30),connected to the primary piston (3), is provided in the annular chamber(27), around the small-section piston (17 b), to collaborate with thehole(s) (29) in the small-section piston.
 5. Braking device according toclaim 3, characterized in that the sealing means consists of a lip seal(30) of lip (32).
 6. Braking device according to claim 4, characterizedin that the sealing means consists of a lip seal (30) of lip (32). 7.Braking device according to claim 1, characterized in that thelarge-section part (17 a) of the stepped piston lies toward the frontand the small-section part (17 b) lies toward the rear.
 8. Brakingdevice according to claim 7, characterized in that the small-sectionpart (17 b) comprises a shoulder (21) against which there may bearaxially a washer (24) acting as a thrust washer for a compression spring(25), the other end of which bears against a stop piece (26) anchored ina housing (22) of the primary piston.
 9. Braking device according toclaim 1, characterized in that the small-section piston (17 b) isextended toward the plunger distributor (10) by a rod (T) of smallerdiameter.
 10. Braking device according to claim 9, characterized in thata gap exists at rest between the rear end of the rod (T) and the plungerdistributor (10).
 11. Braking device according to claim 1, characterizedin that a compression spring (19) bears against the large-section part(17 a) of the stepped piston (17) and against a split ring (20) anchoredin a groove of the bore (18) of the primary piston (3).